In a typical cellular radio system, wireless terminals (also known as mobile stations and/or user equipment units (UEs)) communicate via a radio access network (RAN) to one or more core networks. The radio access network (RAN) covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks may also be called, for example, a “NodeB” (UMTS) or “eNodeB” (LTE). A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. The base stations communicate over the air interface operating on radio frequencies with the user equipment units (UE) within range of the base stations.
In some versions of the radio access network, several base stations are typically connected (e.g., by landlines or microwave) to a controller node (such as a radio network controller (RNC) or a base station controller (BSC)) which supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks.
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the second generation (2G) Global System for Mobile Communications (GSM). UTRAN is essentially a radio access network using wideband code division multiple access for user equipment units (UEs). In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity. Specifications for the Evolved Universal Terrestrial Radio Access Network (E-UTRAN) are defined for the 3rd Generation Partnership Project (3GPP).
The Evolved Universal Terrestrial Radio Access Network (E-UTRAN) comprises the Long Term Evolution (LTE) and System Architecture Evolution (SAE). Long Term Evolution (LTE) is a variant of a 3GPP radio access technology wherein the radio base station nodes are connected to a core network (via Access Gateways, or AGWs) rather than to radio network controller (RNC) nodes. In general, in LTE the functions of a radio network controller (RNC) node are distributed between the radio base stations nodes (eNodeB's in LTE) and AGWs. As such, the radio access network (RAN) of an LTE system has an essentially “flat” architecture comprising radio base station nodes without reporting to radio network controller (RNC) nodes.
The International Telecommunications Union-Radio communications sector (ITU-R) has specified a set of requirements for 4G standards, named the International Mobile Telecommunications Advanced (IMT-Advanced) specification. ITU-R has also stated that Mobile WiMAX and LTE, as well as other beyond-3G technologies that do not fulfill the IMT-Advanced requirements, could nevertheless be considered “4G”, provided they represent forerunners to IMT-Advanced compliant versions and have a substantial level of improvement in performance and capabilities with respect to the initial third generation system.
In order that a network such as 3G LTE system may maintain synchronization and the system may manage the different types of information that is carried between the base station and the UE, a frame structure has been defined. There are two types of LTE frame structures, e.g., Type 1 for LTE frequency division duplex and Type 2 for LTE time division duplex. The basic Type 1 LTE frame has an overall length of 10 ms. This is then divided into a total of 20 individual slots. An LTE subframe has two slots, so that there are ten LTE subframes within a frame. The LTE Type 2 frames are somewhat different: the 10 ms frame comprises two half frames, each 5 ms long. The LTE half-frames are further split into five subframes, each lms long.
Network multiple-input and multiple-output (MIMO) and collaborative MIMO have been proposed for LTE. With a MIMO system, the data stream from a single user is demultiplexed into Ntx separate sub-streams. Each sub-stream is then encoded into channel symbols. A data modulation rate, either same or adaptive, is imposed on the sub-streams of the transmitters. The signals are received by Ntx receive antennas. With a MIMO system consisting of nT transmit antennas and nR receive antennas, the channel matrix is written as shown in Expression (1).
                              H          =                      [                                                                                h                    11                                                                    …                                                                      h                                          1                      ⁢                                              n                        T                                                                                                                                                              h                    21                                                                    …                                                                      h                                          2                      ⁢                                              n                        T                                                                                                                                          …                                                  …                                                  …                                                                                                  h                                                                  n                        R                                            ⁢                      1                                                                                        …                                                                      h                                                                  n                        R                                            ⁢                                              n                        T                                                                                                                  ]                          ⁢                                  ⁢        where                            Expression        ⁢                                  ⁢                  (          1          )                                                  h          ij                =                  α          +          jβ                                    Expression        ⁢                                  ⁢                  (          2          )                                                                  ⁢                  =                                                                      α                  2                                +                                  β                  2                                                      ·                          ⅇ                              j                ⁢                                                                  ⁢                arctan                ⁢                                                                  ⁢                                  β                  α                                                                                        Expression        ⁢                                  ⁢                  (          3          )                                                                  ⁢                  =                                                                  h                ij                                                    ·                                          ⅇ                                  j                  ⁢                                                                          ⁢                                      ϕ                    ij                                                              .                                                          Expression        ⁢                                  ⁢                  (          4          )                    
Indeed, LTE and WiMAX utilize Multiple-Input Multiple-Output (MIMO) transmission schemes to increase spectral efficiency. MIMO schemes assume that the transmitter and receiver are both equipped with multiple antennas, and that multiple modulated and precoded signals are transmitted on the same “time-frequency resource element”. In MIMO technology, mathematically the transmitted signal for a particular frequencytime resource element (k,l) can be expressed by Expression (1).x(k,l)=W(k)s(k,l)  Expression (5)In Expression (5), s is a vector with elements Si, i=1, . . . , Ns, and where Si is a modulated symbol and Ns is the number of transmitted layers; W(k) is the so-called precoding matrix of dimension Ntx×Ns, where Ntx is the number of transmitted antennas; x is a vector of transmitted signals, where xi, i=1, . . . , Ntx, is the signal transmitted from the ith transmit antenna. As used herein, “k” and “l” are the frequency and time indices, respectively, and each element in vectors x and s are given for a particular frequency/time. The signal is transmitted over a channel which can be characterized by a channel matrix H, the channel matrix H being of dimension Nrx×Ntx, where Ntx is the number of transmitted antennas and Nr is the number of received antennas. In general, the rank of the channel matrix is given by rank(H)=k≦min{Nrx,Ntx}. The received signal vector is then an Nrx dimensional vector given by Expression (5).y=Hx+e=HWs+e  Expression (6)In Expression (6), e is a noise and interference vector, with covariance matrix Re.
Factors to consider for MIMO include: geographical separation of antennas, selected coordinated multi-point processing approach (e.g., coherent or non-coherent), and coordinated zone definition (e.g., cell-centric or user-centric). Depending on whether the same data to a UE is shared at different cell sites, collaborative MIMO includes single-cell antenna processing with multi-cell coordination, or multi-cell antenna processing.
High Speed Downlink Packet Access (HSPA) enhances the WCDMA specification with High Speed Downlink Packet Access (HSDPA) in the downlink and Enhanced Dedicated Channel (E-DCH) in the uplink. HSDPA achieves higher data speeds by shifting some of the radio resource coordination and management responsibilities to the base station from the radio network controller. Those responsibilities include one or more of the following: shared channel transmission, higher order modulation, link adaptation, radio channel dependent scheduling, and hybrid-ARQ with soft combining.
High Speed Downlink Packet Access (HSPA) employs a transport channel and three physical channels. The High Speed Downlink Shared Channel (HS-DSCH) is a downlink transport channel shared by several UEs. The HS-DSCH is associated with one downlink DPCH, and one or several physical channels. The following physical channels have been defined for HSDPA: High Speed Physical Downlink Shared Channel (HS-PDSCH); High Speed Dedicated Physical Control Channel (HS-DPCCH); and the High Speed Shared Control Channel (HS-SCCH). The HS-PDSCH is a downlink channel which is both time and code multiplexed. The HS-DPCCH is an uplink channel that carries the acknowledgements of the packet received on HS-PDSCH and also the CQI (Channel Quality Indication). The HS-SCCH is a fixed rate downlink physical channel used to carry downlink signaling related to HS-DSCH transmission. The HS-SCCH provides timing and coding information, thus allowing the UE to listen to the HS-DSCH at the correct time and using the correct codes to allow successful decoding of UE data.
To support the transmission of downlink and uplink transport channels, there is a need for certain associated downlink (DL) control signaling. This control signaling is often referred to as downlink (DL) L1/L2 control signaling, indicating that the corresponding information partly originates from the physical layer (Layer 1) and partly from Layer 2 (Medium Access Control [MAC]). Downlink Al1/L2 control signaling consists of downlink (DL) scheduling assignments, including information required for the terminal to be able to properly receive, demodulate, and decode the DL-SCH on a component carrier, uplink scheduling grants informing the terminal about the resources and transport format to use for uplink (UL-SCH) transmission, and hybrid ARQ acknowledgments and response to UL-SCH transmissions. In addition, the control downlink signaling can also be used for transmission of power-control commands for power control of uplink physical channels, as well as for certain special purposes such as MBSFN notifications. The downlink L1/L2 control signaling is transmitted within the first part of each subframe. Thus, each subframe can be said to be divided into a control region followed by a data region, where the control region corresponds to the part of the subframe in which the L1/L2 control signaling is transmitted. To simplify the overall design, the control region always occupies an integer number of OFDM symbols.
HS-SCCH orders exist in HSPA as a fast L1/L2 control signaling complement to higher layer (Radio Resource Control [RRC]) signaling. By using a special format of the HS-SCCH, it is possible to convey orders to a UE without having to resort to slow higher layer signaling. Currently there are orders specified for (de)activation or triggering of the following features (See section 4.6C in Reference [1] and sections 6A.1, 6B, 6C.4 and 10.5 in Reference [2] for details):                UE DTX (de)activation (orders introduced in Rel-7);        UE DRX (de)activation (orders introduced in Rel-7);        HS-SCCH-less operation (de)activation (orders introduced in Rel-8);        Enhanced serving cell change triggering (orders introduced in Rel-8);        Secondary downlink carrier (de)activation in MC-HSDPA (orders introduced in Rel-8, Rel-9, Rel-10 and Rel-11);        Secondary uplink carrier (de)activation in DC-HSUPA (orders introduced in Rel-9); and        Switching between UL transmit diversity activation states (orders introduced in Rel-11).        
New HS-SCCH orders are being considered within an ongoing Rel-11 work item as indicated in R1-111336. See, e.g., Reference [9]. New HS-SCCH orders may also be considered within other ongoing Rel-11 work items such as described in RP-111393, RP111375 and RP111-642. See, e.g., Reference [3], Reference [10], and Reference [11].
Currently, a four Tx transmissions scheme for HSDPA (high speed downlink packet access) is discussed within 3GPP for standardization. See, e.g., Reference [3]-Reference [5]. Previous versions of the specification support up to 2Tx antenna transmissions from the network side where common pilots (e.g., CPICH) are transmitted from each Tx branch.
To support four Tx MIMO transmissions, it is necessary to obtain four channel estimates in order to characterize each of the spatial layers, which means that more pilots will be necessary. The common pilots are used for two main functions: (1) channel state information (CSI) estimation through channel sounding where rank, CQI and PCI are estimated; and (2) channel estimation for demodulation purposes.
For a four-branch MIMO, the following approaches are possible: (a) common pilots for both CSI and channel estimation for data demodulation; and (b) common pilots for CSI estimation and additional pilots for channel estimation for data demodulation. Sometimes the common pilots for CSI estimation are referred to as the 1st and 2nd common pilots, whereas the “additional” pilots are referred to as the 3rd and 4th common pilots.
As used herein, “common pilots” refer to pilot signals that are made available to all user equipments (UE) and which are transmitted without UE-specific beam forming. Common pilots may be transmitted at instances in which legacy UEs (Release 7 MIMO and Release 99), that are not able to demodulate 4Tx transmissions, are scheduled. These legacy UEs cannot make use of the energy in the 3rd and 4th common pilots. Also the energy made available in the 3rd and 4th pilots reduces the amount of energy available for HS-PDSCH scheduling to the legacy UEs. Moreover, the 3rd and 4th common pilots can cause interference to these legacy UEs, which at best can make use of the 1st and 2nd common pilots. Therefore, to minimize performance impacts to non 4Tx UEs, it is desirable that the power of at least the 3rd and 4th common pilots be reduced to a low value.
A solution based only on common pilots will have a negative impact on the legacy UEs unless the powers on the 3rd and 4th common pilots are minimal. However, if the powers are minimal, then the demodulation performance of 4Tx UEs will be adversely impacted.
FIG. 1 and FIG. 2 show example link level throughputs as a function of pilot powers on 3rd and 4th pilots for a non-legacy UE with three different geometries for 4×4 MIMO and 4×2 MIMO systems. In FIG. 1 and FIG. 2, the pilot powers for the 1st and 2nd pilots are maintained at −10 and −13 dB, respectively. It can be observed, e.g., in FIG. 1 and FIG. 2, that as the 3rd and 4th pilot powers are reduced, the performance of the non-legacy UE degrades. The degradation is severe at a high C/I (e.g., at 20 dB). This is because at high C/I, there is a high probability of rank 3 and rank 4 transmissions and/or high data rates, which require a larger amount of pilot power energy. On the other hand, low data rates and/or rank selections, which occur at low C/I (e.g., 0 dB) can be demodulated with a lower amount of pilot energy. Thus, high pilot power is desirable when a UE is to demodulate high data rates with high rank.
It has been proposed to introduce scheduled pilots, which are additional pilots on the 3rd and 4th antennas transmitted only when a 4-branch capable UE is scheduled. See Reference [7]. Introduction of the additional pilots when any 4 branch MIMO user is scheduled is likely to cost additional overhead without providing benefit for all the scenarios. In reality, a high amount of pilot power is required when the UE is to demodulate high data rates with high rank. But as described above, high pilot powers can negative impact the legacy UEs. The impact can be substantial if the 4-branch UEs are scheduled fairly often.
Another drawback with the additional pilots is that they are transmitted even if a UE does not support high rank (e.g., 3 and 4) signals. It can be expected that not all UEs capable of receiving a 4-branch transmission will support multiplexing of up to 4 layers. In Reference [8], the proposed UE categories are listed. Most likely, there will be 4-branch UEs capable of receiving at most 2 layers (also known as 4×2 MIMO). Since these UEs will not be able to receive the very high bit rate that can be provided if 4 layers are transmitted, the need for the additional pilots for demodulation is not as urgent. Hence it is recommended that Node B use these additional pilots only under certain conditions. See e.g., Reference [6].